Electrochemical capacitors are a class of high-rate energy storage devices which use electrolytes and electrodes of various kinds in a system similar to that of conventional batteries. Electrochemical capacitors, like batteries, are essentially energy storage devices. However, unlike batteries, they rely on charge accumulation at the electrode/electrolyte interface to store energy. Charge storage in electrochemical capacitors therefore is a surface phenomenon. Conversely, charge storage in batteries is a bulk phenomenon occurring within the bulk of the electrode material.
Electrochemical capacitors can generally be divided into one of two subcategories: Double layer capacitors in which the interfacial capacitance at the electrode/electrolyte interface can be modeled as two parallel sheets of charge; and pseudocapacitor devices in which charge transfer between the electrolyte and the electrode occurs over a wide potential range, and is the result of primary, secondary, and tertiary oxidation/reduction reactions between the electrode and the electrolyte. These types of electrochemical capacitors are being developed for high-pulse power applications.
Most of the known pseudocapacitor active materials are based on metal elements such as platinum, iridium, ruthenium, or cobalt. These materials are generally quite expensive and pose a significant hurdle to the wide-spread commercialization of this technology. Moreover, the use of two electrodes fabricated of similar materials in a symmetric configuration and having redox potentials in a relatively narrow voltage range restricts the cell voltage and hence the deliverable energy density. That is, the voltage ranges are small and hence the commercial applicability of the device is limited.
Moreover, most electrode materials known for pseudocapacitor devices have their redox reactions occurring at positive potentials relative to a mercury/mercury oxide (Hg/HgO) reference electrode. That is they are generally only applicable for cathode applications.
The use of two dissimilar electrodes having redox potentials in a wide voltage range extends the cell voltage in the asymmetric configuration and hence leads to higher deliverable energy. The need for new pseudocapacitor active materials that could be used as either anodes with redox reactions occurring at negative potentials or cathodes with redox reactions occurring at positive potentials in asymmetric pseudocapacitors is the driving force to explore materials that are inexpensive in cost, processable and non-toxic in nature.
Accordingly, there exists a need for pseudocapacitive anode materials characterized by redox reactions occurring at negative potentials relative to a Hg/HgO reference electrode in an aqueous electrolyte. Moreover, such materials should be abundant in nature, inexpensive in cost, readily processable into devices, and non-toxic.